Electric motors are present in a variety of appliances. Changes in polarization between the poles of a rotor and a stator create movement of the rotor. In case of brushless motors, the poles of the stator are magnetized by current circulating through a conductor, which forms a coil (winding) around the pole. The electromotive force stems from the change of stator pole polarization, which attracts the rotor pole. The rotor pole may be ferromagnetic (reluctance motor), or may present high coercivity (hysteresis motors) or may use magnets (permanent magnet motors). In general, the rotor poles tend to align with the stator poles, producing a movement in the rotor while the polarization of the poles change. In case of Switched Reluctance Motors (SRM), torque generation is based on the minimization of the reluctance in air gaps between the stator and rotor by aligning the rotor and stator poles. Windings are usually mounted on stator poles. If current flows through opposite windings in the stator, the rotor aligns because the magnetic circuit tries to minimize the air gap between rotor pole and stator pole. There is a wide range of applications, like in ventilators, pumps, engines, etc.
Electric motors can be powered by a direct-current (DC) source, or by other type of current. In general, a power source is connectable to the electric motor. The windings can be powered in commutation, switching the power from one winding to the next. In particular, brushless DC motors can be driven by pulse-width modulation, which typically provides a square signal of constant amplitude which changes its duty cycle at a given frequency. The PWM signal controls for how long the winding is powered. The commutation between windings can produce torque ripples during fast changes in the motor current, which results in audible noise emission. Current must be carefully controlled, because changes in induction, temperature and others may change the impedance of the windings. Current control is usually required in SRM to reduce torque ripple, hence reducing audible noise. Usually some kind of sensor is introduced to allow current control. Torque or position sensors are sometimes introduced, but these are typically expensive. In other cases, a current sensor is introduced in each winding, notably increasing the number of external components and interconnects and the size of the device particularly in those cases in which switching of current sensor is necessary.
A compromise is the introduction of a current sensor that controls the amount of electrical power being introduced in the motor. For instance, document EP0832513 shows a PWM-controlled motor and a current sensor for sensing power supply current flowing in the motor, for feedback to a PWM controller. This solution improves total current control and uses less space than a current control in each winding, but controlling the driver circuit becomes difficult, especially in phase overlapping mode (see for example FIG. 22). In case of failure or change of conductance of the wiring, the control system will not respond properly either. It would be necessary to improve driving control of DC motors, while minimizing the amount of external components introduced in the system.